Flow control technology relates generally to the capability to alter flow properties relative to their natural tendencies by introduction of a constant, or periodic, excitation. Use of a periodic excitation for control of boundary layer separation has been demonstrated to be both possible and efficient in incompressible flows (Seifert et al. (1996) “Delay of Airfoil Stall by Periodic Excitation”, J of Aircraft. Vol. 33, No. 4, pp. 691 699 and Seifert et al. (1999) “Oscillatory Control of Separation at High Reynolds Numbers”, AIAA J. 37(9): 1062-1071) especially at low speeds and in a wide range of Reynolds numbers (Re; 100.4 to 107). Control of boundary layer separation in compressible flows has also been demonstrated, although the level of oscillation required is higher than that required in in-compressible flows (Seifert et al. (2001) “Oscillatory Control of Shock-induced Separation”, (AIAA paper 99 0925), J. Aircraft, 38(3): 464 472 and Seifert et al. (2003) “Effects of Compressibility and Excitation Slot Location on Active Separation Control at High Reynolds Numbers”, J. Aircraft 40 (1): pp. 110-119). Despite this, as long as the flow is free of shock waves, there is no theoretical or physical difference resulting from the mere increase of Mach number. One of the primary uses of flow control in boundary layer control is to delay, prevent or manage unwanted boundary layer separation.
Significant scientific and technological effort has been invested in control of boundary layer separation. Alternate methods of flow actuation have been examined including mechanical mixing (e.g. vortex generators, Allan et al (2002) Numerical Simulations of Vortex Generator Vanes and Jets on a Flat Plate, AIAA Paper 2002 3160), pneumatic vortex generator-jets (e.g., steady and oscillatory, Johnston, et al. (2002) International J. of Heat and Fluid Flow, 23(6):750 757; and Khan and Johnston, (2000) International J. of Heat and Fluid Flow, (21(5): 505 511.) and cyclic excitation. Under certain conditions (e.g. at low Re numbers) that cyclic excitation is more efficient than steady excitation for boundary layer control by about two orders of magnitude (Seifert et al (1996) J. of Aircraft 33(4):691-699).
Prandtl defined the boundary layer and the scientific and engineering advantages to be realized its control. Prandtl also defined the basic theoretical problems related to control of boundary layer separation and went on to explain one possible solution to these problems, control of the boundary layer separation by suction, applied upstream of the separation point with suppression of the negative phenomena resulting from the flow detachment from the surface. These phenomena lead to reduction in efficiency of the flow related mechanism. Prandtl demonstrated the efficacy of boundary layer suction by placement of suction ports upstream to the boundary layer separation point in a wide angle diffuser, whose boundary layers separated without control. In the presence of suction, the flow remained attached to the two walls of the diffuser (Prandtl and Teitjens (1934) Applied Hydro and Aerodynamics; Dover, N.Y.; page 294).
Even in a case where suction of the boundary layer prevents separation locally, downstream spreading of flow streamlines can cause boundary layer separation downstream of the point where suction is applied.
U.S. Pat. No. 7,055,541 to Seifert et al. describes methods and mechanisms for Producing Suction and Periodic Excitation Flow including embodiments in which an exit flow direction oscillates of a boundary layer control fluid stream oscillates. The disclosure of this patent is fully incorporated herein by reference.
It is known to employ a converging-diverging inlet nozzle in conjunction with a suction flow. In an apparatus with a given set of dimensions, a converging-diverging inlet nozzle can function in a variety of ways depending on operational conditions (see FIG. 3; taken from Streeter and Wylie (1981) Fluid Mechanics, 7th Edition; McGraw Hill Ryerson; page 283). FIG. 3 presents converging-diverging nozzle pressure and Mach number characteristics. If the flow at the nozzle exit is desired to be supersonic, it is required to have a nozzle pressure-ratio that will ensure ideally expanded jet (below point j, in FIG. 3). A higher pressure ratio will cause a shock wave right downstream of the nozzle exit and the flow will return to subsonic conditions. When wholly subsonic operation is desired and efficiency is a prime consideration, a short converging inlet nozzle suffices.